Hypothalamic but not pituitary or ovarian defects underlie the reproductive abnormalities in Axl/Tyro3 null mice

AXL and TYRO3, members of the TYRO3, AXL and MER (TAM) family of tyrosine kinase receptors, modulate GnRH neuronal cell migration, survival and gene expression. Axl/Tyro3 null mice exhibit a selective loss of GnRH neurons, delayed sexual maturation and irregular estrous cycles. Here we determined whether the defects were due to direct ovarian defects, altered pituitary sensitivity to GnRH and/or an impaired LH surge mechanism. Ovarian histology and markers of folliculogenesis and atresia as well as corpora luteal development and ovarian response to superovulation were not impaired. Axl/Tryo3 null mice exhibited a robust LH response to exogenous GnRH, suggesting no altered pituitary sensitivity. Ovariectomized Axl/Tyro3 null mice, however, demonstrated an impaired ability to mount a steroid-induced LH surge. Loss of GnRH neurons in Axl/Tyro3 null mice impairs the sex hormone-induced gonadotropin surge resulting in estrous cycle abnormalities confirming that TAM family members contribute to normal female reproductive function.